Roblox-
Pro-
Ingame scripting, potential for a much better system of players when regulated correctly, easier building, interactive environments, and buildable vehicles
Cons-
Game is targeted towards younger audience, and community is overall immature
Engine isn't suited for massive physics events. 350 bucks is the best buy for "builders club" which you can pretty much get around.
Blockland-
Pro-
Much better gameplay, engine may be old but can handle more than Roblox, and the community is much more mature, and the game is targeted towards everyone.
Also provides a window into game dev(scripting, modeling, etc.) Costs 20 bucks for lifetime membership
Cons-
Look in drama section, no ingame modeling/scripting(?). Closest to scripting is eventing, which is actually somewhat better in that you can get simple tasks done faster.